1. Field of the Invention
The present invention relates to image forming apparatuses, such as facsimiles, printers, copiers, etc., and in particular to image forming apparatuses capable of transferring a visual image formed on an image bearer to a moving member such as a transfer belt at a position at which the image bearer contacts the moving member.
2. Background of the Invention
A background color image forming apparatus forms a color image by superposing component color (i.e., Y, M, C, and K) images formed on respective photoconductive (PC) drums using a laser light. This method sometimes raises a considerable problem. That is, respective distances and parallelism between shafts of the PC drums are, at times, unequal. An error occurs in setting a deflection mirror, which deflects a laser light irradiated from an exposure instrument. Occasionally the time varies when a latent image is written to each of the respective PC drums. As a result, superposed toner images do not fully coincide with a prescribed position, and displacement takes place between the respective color component toner images. Various countermeasures have been developed to help solve this problem.
For example, as shown in FIG. 14, a background image forming apparatus attempts to stabilize the traveling performance of a conveyance belt by detecting and correcting dynamic deviation in the belt moving speed as an AC component. Such an image forming apparatus includes a rotary encoder 20, an FV converter 22, a filter 23, a comparator 25, a motor driver 26, and a conveyance belt driving motor 27. The FV converter 22 converts a square-wave pulse train detected by the rotary encoder 20 into a voltage signal. A shaft vibration, for example, caused by an error in assembling the rotary encoder 20 to a driven roller 8A is superposed as a noise on such a voltage signal. The filter 23 then filters out the shaft vibration occurring at a specific frequency, and the comparator 25 calculates a difference between the voltage signal and a reference speed instruction signal 24.
The comparator 25 then outputs a control signal to a motor driver 26 in accordance with the difference. The motor driver 26 controls the rotational speed of the conveyance belt driving motor 27 in accordance-with the control signal upon its input.
Further, as discussed in Japanese Patent Application Laid Open No. 9-267946, to suppress the above-mentioned displacement, an angular speed of a driven roller rotating in synchronism with an intermediate transfer belt is detected and a filter device filters out a rotational deviation of the driven roller occurring at a predetermined frequency from the detected angular speed deviation. Then, the belt traveling speed can be controlled in accordance with the filtered angular speed deviation.
Further, according to a color image forming apparatus described in Japanese Patent Registration No. 3153331, the displacement is suppressed by detecting unevenness and controlling a rotational speed of a PC drum driving motor based upon an alternate current component included in a current wave of the PC driving motor.
However, since the filter device employed in the above-noted art is hardware based and formed from an LC circuit and similar devices, it causes unevenness of filtering performance. Thus, the background art devices have poor frequency performance due to a fluctuation factor, such as unevenness of parts and operational temperature, elapsing of time, etc. As a result, the belt cannot predictably travel at a prescribed frequency. To resolve such a problem, parts of the filter require a higher level of precision. However, with the introduction of more precise components the cost necessarily increases.